*{
    margin: 0;
    padding: 0;
}
body{
    background-color: #2c3e50;
}
#app{
    width: 90vw;
    height: 100vh;
    margin: auto;
    background-color: #2c3e50;
    padding: 20px;
    color: #ffffff;
}
.retu{
    display: block;
    width: 90px;
    height: 20px;
    background-color: #95a5a6;
    color: #ffffff;
    line-height: 20px;
    cursor: pointer;
    text-decoration: none;
    text-indent: 15px;
}
.retu:hover{
    background-color: #2980b9;
}